1. Field of the Invention
The present invention relates to an image forming apparatus of the kind using an electrophotographic method or an electrostatic recording method, such as an electrophotographic copying machine, a facsimile set, a printer or the like, and more particularly, to the method for cleaning a fixing device used for such an image forming apparatus.
2. Description of Related Art
Image forming apparatuses such as electrophotographic copying machines are generally provided with a web cleaning device having a cleaning web which is made of a nonwoven cloth or the like for cleaning and removing a residual toner left on an object to be cleaned such as a fixing roller.
FIG. 6 shows, by way of example, a conventional fixing device arranged to fix, by heating, a toner image T formed by the electrophotographic method to a transfer member P which is employed as a recording member. The fixing device shown in FIG. 6 is provided with a fixing roller 101 which is a fixing rotary body having a heating heater 103 such as a halogen heater disposed inside thereof. The fixing roller 101 is arranged to be rotated in the direction of an arrow by a driving force of a driving motor (not shown).
A pressure roller 102, which is supported by a bearing (not shown) below the fixing roller 101, is caused by some known pressing means to be in pressed contact with the fixing roller 101 at least in carrying out a fixing process. The rollers 101 and 102 are thus arranged to rotate in a state of being in pressed contact with each other.
A thermo-sensitive element 104 such as a thermistor or a thermocouple is disposed in contact with the outer circumferential surface of the fixing roller 101. The thermo-sensitive element 104 is thus arranged to detect the temperature of the fixing roller 101 and to send a detection signal to some known control means. The control means is arranged to control the output of the heater 103 or a voltage applied to the heater 103 on the basis of the detection signal of the thermo-sensitive element 104, so that the outer circumferential surface of the fixing roller 101 is kept at a toner-image fusing temperature.
Separation claws 151 and 152 are disposed in contact respectively with the fixing roller 101 and the pressure roller 102 and are arranged to reliably separate the transfer member P from the rollers 101 and 102 after a fixing process. The separation claws 151 and 152 are made of a heat resisting resin, such as a polyimide or polyamide resin, coated with a fluororesin.
Meanwhile, a cleaning device 106 is disposed above the fixing roller 101 to remove an offset toner and a foreign matter such as paper powder or the like sticking to the surface of the fixing roller 101. The cleaning device 106 uses a cleaning web (hereinafter referred to simply as the web) 160 which is a band-shaped element made of a heat resisting nonwoven cloth, such as Nomex (trade name), Himelon (trade name), etc.
Further, a web pressing roller 165 is rotatably supported by a bearing 172 at each of its two ends. The offset toner and foreign matter such as paper powder or the like sticking to the surface of the fixing roller 101 are removed from the surface of the fixing roller 101 and are thus effectively prevented from offsetting, with the web pressing roller 165 arranged to be in pressed contact with the fixing roller 101.
Generally, the web 160 is arranged to be wound, or taken up, as much as a predetermined amount per sheet of copy by a driving means (not shown), irrespective as to whether the image forming apparatus is in the mode of copying (printing) a single sheet or in the mode of continuously copying a plurality of sheets.
The above-stated fixing device has almost no offset caused by slipping-off of an offset toner from the web 160 while copying (printing) is in process. This is because only a very small amount of toner slips off being not captured by the web 160 in the process of copying. Besides, the slip-off toner, coming into contact with both the fixing roller 101 and the pressure roller 102, is dispersed on the surfaces of both the fixing roller 101 and the pressure roller 102 to evenly flow out little by little to the surface and back sides of the transfer member P during the copying process. The slip-off toner is, therefore, hardly notable while copying is in process.
On the other hand, when the rollers 101 and 102 are caused to rotate immediately after the start of the image forming apparatus, or when copying is resumed after a pause, however, some residual toner is not completely captured by the web 160 and is allowed to slip off. The slip-off toner then might come out in lumps to stick to the recording member, i.e., the transfer member P, to conspicuously smudge the recording member. Further, if the slip-off toner on the fixing roller 101 comes into contact with the pressure roller 102, the slip-off toner becomes solid on the pressure roller 102 to stick to the pressure roller 102, because the temperature of the pressure roller 102 is relatively low at this time. A major portion of the slip-off toner is thus transferred from the fixing roller 101 to the pressure roller 102.
When the transfer member P is fed under this condition, the majority of the slip-off toner on the pressure roller 102 is re-transferred to the back side of the transfer member P, which is inferior in parting property than the pressure roller 102. Therefore, a back-side smudged state of the transfer member P tends to stand out.
The smudge which results from offsetting becomes salient particularly when copying is restarted after a pause after completion of continuous copying, due to the following reason.
The fixing roller 101 is controlled and adjusted to be at a predetermined fixing temperature. However, in the case of continuous copying, the process of adjusting the temperature of the fixing roller 101 to the fixing temperature might gradually become too slow, depending on the number of sheets in continuous copying. The temperature of the fixing roller 101 then drops to deteriorate the fixing power of the fixing roller 101. As a result, the amount of toner offsetting on the surface of the fixing roller 101 comes to increase.
Further, in carrying out continuous copying to obtain a great number of copies of one and the same image, the offset toner concentrates in a position corresponding to the image. This also causes the offsetting smudge to become conspicuous after continuous copying.
The above phenomenon becomes conspicuous also in a case where the operation of the image forming apparatus is restarted after using such transfer members that are inferior in fixing property and thus tend to increase the amount of residual toner.
It is an object of the invention to provide an image forming apparatus which is capable of preventing a recording member from being smudged by some offset toner when an image forming operation is performed after a pause after completion of a continuous image forming operation.
It is another object of the invention to provide an image forming apparatus in which, in a case where the number of recording members during a continuous image forming mode is not less than a predetermined number, a cleaning member moves by a second amount larger than a first amount after completion of a continuous image forming operation.
It is a further object of the invention to provide an image forming apparatus in which, after completion of an image forming operation, a cleaning member moves by an amount not less than the width of a part where a rotary member and the cleaning member are in contact with each other in a moving direction of the cleaning member.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.